1. Field of the Invention
This invention relates to a stable pesticidal aqueous suspension of an N-methyl- or N-phenylcarbamate as a pesticidally active ingredient.
2. Description of the Prior Art
Conventional pesticidal formulations which are applied as aqueous dilutions consist of an emulsifiable concentrate, a wettable powder and a suspension (or a flowable formulation) which have their own advantages and are therefore used for different purposes. A suspension, especially an aqueous suspension, is considered more adaptable and advantageous than the other two formulations because it allows substituting water for an organic solvent. An organic solvent poses a potential harm to beneficial animals and plants and often presents safety problems in manufacture, storage, transportation and field application. An aqueous suspension allows all operations from manufacture to field application to be accomplished in a wet system, thus eliminating the disadvantages caused by the drifting of the dusty particles of the formulation, and it can be prepared at lower cost from a less expensive materials. However, the suspension is afflicted with a serious problem with retention of its physical properties that prevents its expanded use. As the formulation is stored for an extended period, its dispersed particles grow larger or separate from the dispersion medium to form sediments. It therefore would be a great contribution to the art if this problem could be solved and a stable aqueous suspension were provided.
The term "pesticidal aqueous suspension" used herein means a composition which has fine particles of a pesticidally active ingredient dispersed in water together with dispersion agents. Such composition is prepared by various conventional methods such as by using an air mill or a hammer mill to make fine particles of a solid pesticide which is then dispersed in water with a dispersing agent, or by adding a solid pesticide to a dispersion agent containing water and dispersing the pesticide by making fine particles thereof with the use of a wet grinder such as a comminutor, ball mill, oscillating mill, tower mill, attritor, colloid mill or sand mill, as is disclosed in Japanese Patent Publication No. 20519/71; or by forming a solution of the solid pesticide in a hydrophilic solvent and adding the solution into water to crystallize fine particles of the pesticide suitable for dispersion in water, as is disclosed in Japanese Patent Publication No. 38150/73.
The thus prepared suspension cannot maintain its physical properties stably over an extended period and therefore cannot be used as a commercial product without suitable treatments. Several methods have therefore been proposed to improve the stability of the physical properties of such conventional suspension; for example, controlling the specific gravity of the disperse phase or dispersion medium, or controlling the viscosity of the suspension system, or preventing formation of a layer of a hard sediment (hard cake) to assure easy re-dispersing at the time of field application. Specific examples of these conventional methods are: using finely ground particles of a petroleum resin, coumarone resin or ester gum together with a water soluble polymer to provide the mixture with suitable viscosity and thixotropic properties, thus preventing deterioration of the physical properties during storage (see Japanese Patent Publication No. 148625/77); use of colloidal silicone and other agents that assure resuspending (see Japanese Patent Publication (OPI) No. 52918/73) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"); addition of a dye of a metal complex compound to prevent crystallization of an aqueous dispersion (see Japanese Patent Application (OPI) No. 126635/74); using a concentrated aqueous solution of ammonium sulfate or sodium hydrogenphosphate to provide a stable suspension system (Japanese Patent Application (OPI) No. 76236/75); adding a water soluble polymer to provide a homogeneous suspension having a viscosity between 200 to 500 cps (see Japanese Patent Application (OPI) No. 66633/77 and U.S. Pat. No. 4,071,617); stabilization of a dispersion system by addition of a hetero polysaccharide gum (xanthane gum) as disclosed in Japanese Patent Application (OPI) No. 128226/77; and stabilization of a dispersion system by addition of urea or fats, as disclosed in U.S. Pat. Nos. 2,957,803 and 3,184,380.
Generally, the physical properties of a suspension system are deteriorated due to settling, agglomeration, and growth of dispersed particles, and the prevention of the last-mentioned phenomenon is most difficult to accomplish and this is particularly prominent when a very slightly water soluble pesticidally active ingredient is dispersed in water. A pesticidally active ingredient, especially a carbamate compound, has a solubility in water generally in the range of from several tens to several thousands ppm, and a suspension of such a very slightly soluble compound is particularly low in terms of the stability of its physical properties. While an aqueous suspension generally comprises finely dispersed particles of a size in the range of from 3 to 10 microns, the dispersed particles of such "very slightly soluble compound" gradually grow larger, sometimes to a size larger than 100 microns or even several millimeters, as the dispersion is stored at room temperature for a period of 1 to 2 years. With such big particles, the suspension system not only has its stability deteriorated but also it is substantially unsuitable for field application in respect of sprayability, effectiveness or phytotoxicity to plants.